An optical coupler/splitter for branching and coupling optical signals between optical transmission lines is an essential device for configuring an optical communication network. To improve operation reliability of an optical device and make it compact in size, it is known to use, as an optical coupler/splitter, a directional coupler which has an optical coupling section consisting of two optical waveguides arranged in parallel to each other so as to achieve evanescent-field coupling therebetween. The waveguide type directional coupler, however, is disadvantageous in that its efficiency of optical coupling between optical waveguides heavily depends on the wavelength of light. In an optical coupler/splitter, high dependence of the coupling efficiency on wavelength makes it difficult to branch or couple optical signals at a specified ratio, if the wavelength of an optical signal fluctuates due to some reason, or to provide a general-purpose optical coupler/splitter suited for both optical communication networks which are different in working wavelength (e.g., 1.3 .mu.m and 1.55 .mu.m).
To reduce the dependence of the coupling efficiency in a waveguide type directional coupler on wavelength, it has been suggested that the working principle of a Mach-Zehnder interferometer be applied. Typically, this waveguide type optical component has two optical coupling sections, and a phase-shifting section which is provided between the two optical coupling sections and which consists of a curved optical waveguide and a linear optical waveguide. A first optical waveguide is configured by the optical waveguides of one side of the two optical coupling sections and the curved optical waveguide of the phase-shifting section connected between them, and a second optical waveguide of the optical component is configured by the optical waveguides of the other side of two optical coupling sections and the linear optical waveguide of the phase-shifting section connected between them. The optical component according to the above-mentioned suggestion is designed so that the efficiency of coupling between the first optical waveguide and the second optical waveguide stays almost constant in a certain light wavelength range by setting the path difference between the two optical waveguides of the phase-shifting section to a required value and by adjusting parameters (e.g., coupling section length and waveguide space) of each coupling section. However, setting the path difference between the two optical waveguides of the phase-shifting section to a required value unavoidably results in an increased length of the phase-shifting section because it is difficult to decrease the radius of the curvature of the curved optical waveguide. For this reason, the optical component suggested above is disadvantageous in that the optical component becomes large in size. If curved waveguides are used for the two waveguides of the phase-shifting section, then the resulting waveguides will be even longer, causing inconvenience.
It is also known that an optical component, which includes two linear optical waveguides having different propagation constants, is used to eliminate the dependence of the coupling efficiency of a directional coupler on wavelength. While this optical component is advantageous in that the optical waveguides can be made shorter because the coupling sections are linear, it is disadvantageous in that the coupling efficiency considerably varies with changes in light wavelength, and therefore, the flatness of the coupling efficiency (in other words, the dependence of the coupling efficiency on wavelength) is worse than that of the optical component suggested above.